During the fifth project year attention will be directed toward the changes which have taken place after six weeks of acclimatization to hypobaric hypoxia at 17,500 feet. In sixteen individuals, capable of sustained exertion at this altitude, we will study (a) electro-encephalographic changes at rest, asleep, after hyperventilation and after 100% oxygen administration; (b) plasma levels of glucose, lactate, free fatty acids, insulin, glucagon, human growth hormone, thyroxine, prolactin, cortisol, and renin-in basal state, during, immediately after and ten minutes after quantified work on a bicycle ergometer and during sleep; (c) arterial blood gases and pH before and during exertion; (d) ventilatory characteristics and nitrogen washout curves. These studies are designed to supplement similar studies of acutely exposed individuals at this altitude, which we have conducted over the past eight years. Baseline studies will also be done before and immediately after residence at the high altitude.